Certain synthetic carriers are known, for example, U.S. Pat. No. 4,426,433 discloses a carrier with a binder and a powder of a magnetizable material dispersed therein, and carbon black. The resin binder includes styrene butadiene polymers, and the magnetite can be MAPICO BLACK™. Also, U.S. Pat. No. 5,663,027 discloses a carrier of a binder resin, such as a polyester, or a styrene/acrylic copolymer, and a magnetite such as FeO.Fe2O3. In U.S. Pat. No. 4,565,765, there is illustrated a carrier composition comprised of a resin binder of for example, polyamides, epoxies, polyurethanes, polyesters, styrene acrylates, and magnetites like MAPICO BLACKS™. Carbon black can also be included in the carrier according to the disclosure of U.S. Pat. No. 4,565,765. Moreover, in U.S. Pat. No. 5,629,119 there is disclosed melt kneading processes for the preparation of a two component binder type magnetic carrier comprised of a magnetic powder and a binder resin wherein the carrier selected contains therein a release agent.
There are disclosed in U.S. Pat. No. 4,565,765 processes for the preparation of synthetic carriers containing a MAPICO BLACK™ magnetite up to 60 percent by weight of carrier, and VULCAN XC72R™ carbon black up to 8 percent by weight of carrier. The compositions can be ground in a Fitzmill and screened to an average particle size of about 75 microns. The MAPICO BLACK™ magnetite disclosed in U.S. Pat. No. 4,565,765 has a coercivity less than 200 gauss, and therefore is considered soft magnetic. To prepare a hard magnetic carrier, there is selected a hard magnetic powder such as, for example, strontium ferrite which is more insulative than MAPICO BLACK™ magnetite. The induced magnetic moment of a synthetic carrier in an applied magnetic field is a function of the concentration of magnetic material in the carrier particle. It is, therefore, preferred to maximize the amount of magnetic material contained in the carrier particle.
In conductive carriers, it may be desirable to have a conductive binder resin, that is, wherein the binder resin contains sufficient amounts of a conductive additive such as, for example, conductive carbon black, to render the carrier particle conductive. In U.S. Pat. No. 4,565,765, conductive carbon black concentrations of about 8 percent by weight of carrier are disclosed. Together with the MAPICO BLACK™ magnetite, this level of carbon black renders the carrier particle conductive. Possibly affecting the conductivity of the carrier is how the carbon black is dispersed in the binder resin. If the conductive material, such as carbon black, is dispersed too finely, then the carrier conductivity will be lower than if the level of dispersion is more moderate.
Conductive, magnetic synthetic carriers can be used in electrophotographic printers and copiers to form a two-component mixture of carrier and toner that is triboelectrically charged for the development of electrostatic images. However, present methods of manufacturing synthetic carriers require a high percentage loading of carbon black to achieve sufficient conductivity of the developer. This high loading tends to preclude the control of the triboelectric charging properties since the high carbon black loading dominates the surface properties. Present synthetic carriers have low density but do not have the desired high conductivity with acceptable triboelectric charging. Known synthetic carriers can provide reasonable triboelectric charging but the conductivity is 10−10 S/cm, which may be unacceptable in many development systems.
Furthermore, known high-density carriers composed of ferrites or metals such as steel are highly abusive to toners in a development system, which may cause severe developer degradation over time. Specifically, the toner is so abused that its adhesion and triboelectric charging properties can be degraded. The toner abuse increases for low document area coverage, in which case toner residing in the developer housing for an extended time can be subjected to considerable mechanical abuse.
Thus, a low-density synthetic carrier having high conductivity and acceptable triboelectric charging that decreases toner abuse in development systems is still desired.